In the recent semiconductor processing technology, the trend toward higher integration of large-scale integrated circuits, particularly, places an increasing demand for miniaturization of circuit patterns. There are increasing demands for further reduction in size of circuit-constructing wiring patterns and for miniaturization of contact hole patterns for cell-constructing inter-layer connections. As a consequence, in the manufacture of circuit pattern-bearing photomasks for use in the photolithography of forming such wiring patterns and contact hole patterns, also, a technique which enables accurate writing of finer circuit patterns is needed to meet the miniaturization demand.
In order to form a higher accuracy photomask pattern on a photomask substrate, it is of first priority to form a high accuracy resist pattern on a photomask blank. Since the photolithography used in actually processing semiconductor substrates involves reduction projection, the photomask pattern has a size of about four times the actually needed pattern size, but the accuracy requirement is not moderated accordingly. The photomask serving as an original is rather required to have a higher accuracy than the pattern accuracy to be attained after exposure.
Further, in the currently prevailing lithography, a circuit pattern to be written has a feature size far smaller than the wavelength of light used. If a photomask pattern which is a mere four-time magnification of the circuit feature is used, a shape corresponding exactly to the photomask pattern is not transferred to the resist film, due to the influences of optical interference or the like occurring in the actual photolithography operation. To mitigate these influences, in some cases, the photomask pattern is required to be designed to a shape which is more complex than the actual circuit pattern, i.e., a shape which is obtained by application of the so-called optical proximity correction (OPC) or the like. Thus, at present, the lithography technology for obtaining photomask patterns also requires a higher accuracy processing method. The lithographic performance is sometimes represented by a maximum resolution. As to the resolution limit, the lithography involved in the photomask processing step is required to have a maximum resolution accuracy which is equal to or greater than the resolution limit necessary for the photolithography involved in the semiconductor processing step using a photomask.
From a photomask blank having an optical film (e.g., light-shielding film or phase shift film) on a transparent substrate, a photomask pattern is generally formed by coating a photoresist film on the blank, writing a pattern using electron beam, and developing to form a resist pattern. Using the resulting resist pattern as an etch mask, the optical film is then etched into an optical film pattern. In an attempt to miniaturize the optical film pattern, if processing is carried out while maintaining the thickness of the resist film at the same level as in the art prior to the miniaturization attempt, the ratio of film thickness to feature width, known as aspect ratio, becomes higher. As a result, the resist pattern profile is degraded to prevent effective pattern transfer, and in some cases, resist pattern collapse or peeling can occur. Therefore, the thickness of resist film needs to be reduced in accordance with the degree of miniaturization. However, as the resist film becomes thinner, the resist pattern is more susceptible to damage during dry etching of the optical film, undesirably resulting in a lowering of dimensional accuracy of a transferred pattern.
One known method of producing a high accuracy photomask using a thinner resist film involves forming a film separate from the optical film (e.g., light-shielding film or halftone phase shift film) as a processing-aid film. Specifically, a hard mask film is formed between the resist film and the optical film, the resist pattern is transferred to the hard mask film, and dry etching of the optical film is then carried out using the resulting hard mask pattern. JP-A 2007-241060 (Patent Document 1) discloses an exemplary method by which to form a finer pattern. With the intention to establish finer photolithography technology, a light-shielding film is formed of a material of a transition metal and silicon compound capable of shielding ArF excimer laser light despite a thinner film, and a chromium-based material film is used as the hard mask film for processing the light-shielding film, whereby higher accuracy processing becomes possible. Also, JP-A 2010-237499 (Patent Document 2) discloses a photomask of similar construction to Patent Document 1 wherein the hard mask film is of multilayer structure such that the stress introduced during deposition thereof may be mitigated for thereby preventing any drop of processing accuracy during preparation of a photomask.